ELAICH - Educational Linkage Approach in Cultural Heritage. For more information and presentations, please visit: http://elaich.technion.ac.il/ Hands on materials (2): how can we measure the colours?

1. Educational Linkage Approach In Cultural Heritage
E x e r c i s e 5.B
Basic Course
Module 5 Monitoring & Maintenance
Topic 5.1 To Monitor or not to Monitor, that is the question!
Exercise 5.B
Hands on materials: how can we measure the colours?
Authors: Prof. Guido Biscontin, Dr. Francesca Caterina Izzo, Prof. Elisabetta Zendri, Ms. Laura Falchi- Ca’
Foscari University of Venice
Objective
The aim of the exercise is to help students in understanding how the
colorimetric CIE L*a*b* space works, and how it is possible to give an
objective description of an individual perception as colours are.
Introduction
“Monitoring means to carry out evaluation tests at the end of the
conservation intervention and over time to control color changes”
Colour measurements provide an excellent tool to describe the evolution of the
deterioration of the work over time, pointing to the colour change, the
emergence of various forms of degradation.
Do you remember?!
• L * defines the brightness
• a * indicates the value between red and green: a *
positive values appear reddish, negative a * values
greenish.
• b * indicates the value between yellow and blue:
positive b * values appear yellowish, negative b *
values bluish.
Fig. 1 Coordinates of the CIE L*a*b* colorimetric
space
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2. Educational Linkage Approach In Cultural Heritage
E x e r c i s e 5.B
Guidelines
Exercise 5.B should be done after after the Topic 5.1 of the Module 5 “Monitoring and
Maintenance”.
Duration: 30 minutes
Type of activity: it is an individual activity that should be followed by a in class discussion
under the guidance of the teacher.
Before the activity: prepare the copies of the exercise.
Expected results
The students should gain a better knowledge of the colours definition and of the colours
measurements, understanding that colours are not only subjective sensations but can be
defined also objectively.
Exercise 5:B part 1. Consider the following colorimetric data measured on different samples (A, B and
C). Sign the correct position of the point in the CIE L*a*b* space.
sample L a* b*
A 41 40 12
B 18 20 4
C -26 30 86
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3. Educational Linkage Approach In Cultural Heritage
E x e r c i s e 5.B
1) What colour have you obtained (yellow,
red, blue, green, brown...)?
A=
B=
C=
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4. Educational Linkage Approach In Cultural Heritage
E x e r c i s e 5.B
Exercise 5:B part 2. Calculate ∆E from the data reported in the following table.
Remember!!
∆E indicates the chromatic variation (= the difference between 2 colours!) and it is calculated as
follows: ∆E=√(∆L2+ ∆a2+ ∆b2)
∆
In Conservation Science, we normally say that
∆E <5 is acceptable (the colour doesn’t change so much)
∆E > 5 is not acceptable (the colour change is too big)
samples ∆L ∆a* ∆b* ∆E
A 41 40 12
B 18 20 4
C -26 30 86
Do you think the differences between the samples are big or not?
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5. Educational Linkage Approach In Cultural Heritage
E x e r c i s e 5.B
Exercise 5:B part 3. Consider the following colorimetric data measured after a conservation treatment
expressed as difference between the same sample before (not reported in the table) and after the
treatment(samples A, B, C, D)
samples ∆E ∆L ∆a* ∆b*
A 2.7 -2.63 0.25 0.56
B 1.72 -1.68 0.16 0.33
C 1.77 -1.77 0.03 -0.11
D 14.02 3.50 -1.23 -0.51
1) Is sample B more reddish than sample C?
2) Is sample A more yellowish than the others?
3) Is sample A more yellowish than D?
4) Do you think a protective treatment can change the colour of a surface, why?
5) Do you think the colorimetric data reported differ too much from the original sample (look at
the ∆E values)? Are these changes acceptable?
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